The present invention relates to a headbox for a paper machine, the headbox having at least one distributor for distributing the pulp suspension over the width of the machine, a turbulence insert for producing microturbulence, and a headbox nozzle for producing a jet of pulp for applying the pulp suspension onto a wire or between two wires.
Headboxes are known, for instance, from the following documents, which are incorporated by reference:
(1) DE 35 14 554 A1
(2) U.S. Pat. No. 4,909,904
(3) U.S. Pat. No. 5,196,091
In document (1) a headbox is shown having a plurality of feed and/or discharge lines distributed over the width of the machine by which pulp suspension can be withdrawn from the headbox or whitewater can be introduced into the headbox. No detailed information is given in that document as to the manner in which the lines are arranged or, in particular, their angle with respect to the direction of pulp or water flow at the corresponding place in the headbox.
Document (2) also shows a headbox into which dilution water can be introduced, distributed over the width of the machine in front of the turbulence insert, by means of a plurality of tubes. This document emphasizes that the dilution water is introduced parallel to the main direction of flow at the corresponding point of the headbox.
Document (3) also shows a headbox into which dilution water is introduced through a plurality of feed lines, emphasis being placed in this document on the fact that the introduction of the dilution water is effected perpendicular to the direction of flow of the pulp suspension at the corresponding place, and furthermore on the fact that the dilution water is introduced directly in front of the turbulence insert.
Experiments have shown that when suspension or dilution water is introduced into the headbox as disclosed in these documents, e.g. document (3), restricted cross-sections, providing very high resistances to flow, are required to be arranged behind the place of introduction, in order to prevent the introduced suspension or dilution water from disturbing the cross-machine profile of the fiber orientation when correcting the transverse profile of the basis weight.